The present invention relates to a receiving apparatus and particularly to a receiving apparatus that performs communication of code division multiple access (hereinafter referred to as xe2x80x9cCDMAxe2x80x9d) and a synchronization capturing method.
In communication using CDMA, the receiving apparatus provides despread processing to the signal, which has been spread using a spreading code at the transmitting apparatus and transmitted therefrom, at the same phase as that of the transmitting apparatus, whereby allowing the signal to be demodulated. Therefore, for synchronization of data demodulation, it is necessary to detect with what timing the received signal should be multiplied by the known spreading code before performing data communication.
The received signal is multiplied by the known spreading code as shifting the phase. Then, it is detected how much degree the phase is shifted to obtain cross correlation. This processing is called synchronization capturing. In some cases, synchronization capturing is provided to a plurality of transmitting apparatuses. The conventional receiving apparatus that performs such synchronization capturing will be described with reference to FIG. 1.
FIG. 1 is a block diagram showing the configuration of a conventional CDMA receiving apparatus. In FIG. 1, signals transmitted from the respective transmitting stations are received via an antenna 1301 and outputted to a radio receiving section 1302. The signals received via the antenna 1301 are converted to received baseband signals at the radio receiving section 1302. This received baseband signals are outputted to a correlation calculating section 1303.
In the correlation calculating section 1303, the received baseband signals are subjected to despread processing using the same known spreading code as one that is used in the spread processing at the transmission station, whereby calculating correlation values. As a correlation calculating section 1303, a matched filter, a sliding correlator and the like are conventionally known.
The correlation values calculated by the correlation calculating section 1303 are outputted to a delay profile generating section 1304. In the delay profile generating section 1304, the above correlation values are subjected to average processing to suppress noise components contained in the correlation values outputted from the correlation calculating section 1303, whereby a delay profile is generated. The generated delay profile is outputted to a peak detecting section 1305.
The peak detecting section 1305 detects a maximum value (hereinafter referred to as xe2x80x9cpeak correlation valuexe2x80x9d) of the correlation values averaged in the delay profile outputted from the delay profile generating section 1304. Also, a phase (hereinafter referred to as xe2x80x9cpeak phasexe2x80x9d) of the detected peak correlation value of the delay profile is outputted to a demodulation control section 1306. Despread processing is provided to the received signals in accordance with this peak phase. This makes it possible to demodulate the signals transmitted from the respective transmitting stations.
The demodulation control section 1306 stores the peak value outputted from the peak detecting section 1305. Also, a timing signal for demodulating the signal transmitted from each transmitting station is outputted to a baseband demodulating section 1307. The baseband demodulating section 1307 provides despread processing to the received baseband signal outputted from the radio receiving section 1302 based on the timing signal outputted from the demodulating control section 1306, with the result that demodulated data corresponding to each transmitting station is obtained.
However, in the conventional CDMA receiving apparatus, the correlation values calculated using the known spreading code contain influences of auto-correlation components of this spreading code. This causes a problem in which the peak phase corresponding to each transmitting station is not correctly detected for various kinds of factors set forth below. Here, the following will explain an example showing a case in which synchronization capturing is provided to the signals transmitted from the respective transmitting stations with different timing. In this case, the transmitting station, which can obtain the highest reception field intensity by use of the receiving apparatus, is referred to as xe2x80x9cfirst transmitting stationxe2x80x9d and the transmitting station, which can obtain reception field intensity lower than that of the first transmitting station, is referred to as xe2x80x9csecond transmitting station.xe2x80x9d
First, at the time of detecting the peak phase corresponding to the second transmitting station, the reception field intensity of the signal from the second transmitting station is lower than that of the signal from the first transmitting station. For this reason, the original peak correlation value of the second transmitting station is smaller than the auto-correlation component in the signal from the first transmitting station in some instances. In such a case, there is a possibility that the auto-correlation component will be detected as the peak correlation value of the second transmitting station. As a result, the peak value of the second transmitting station may not be correctly detected.
Moreover, at the time of detecting the peak phase corresponding to a certain transmitting station (the first transmitting station, as one example), it is assumed that the reception field intensity of the signal from the first transmitting station is changed during the generation of delay profile. In this case, there is a possibility that the auto-correlation component of the signal from the first transmitting station or the other station will become larger than the original peak correlation value of the first transmitting station. In some cases, other erroneous peak phases are detected as the peak phase of the first transmitting station.
Still moreover, there is a case in which the timing of the peak correlation value of the second transmitting station matches the timing of an odd correlation component, that is, a negative auto-correlation component of the signal from the first transmitting station. In this case, the entire correlation value with this timing is observed as a small value, with the result that other erroneous peak phases are detected as the peak phase of the second transmitting station in some instances.
As mentioned above, for various kinds of factors resulting from the auto-correlation components of the spreading code, there is a possibility that the original peak phase will not be correctly detected as the peak value corresponding to each transmitting station. This reduces accuracy of obtainable demodulation data.
As a receiving apparatus for solving the aforementioned problem, an apparatus that is described in Unexamined Japanese Patent Application No. 10-51504 is known. This apparatus eliminates the correlation components of an interference station from the received signal using the technique of a decorrelater. However, there is a necessity to calculate the correlation component of the interference station from the received signal, causing the problem in which the number of calculations is increased.
It is an object of the present invention is to provide a receiving apparatus, which is capable of detecting an original peak phase corresponding to each transmitting station with a small number of calculations without being subjected to the auto-correlation component of a spreading code included in correlation values.
The auto-correlation component of a transmitting station in a delay profile is calculated using the auto-correlation value of the known spreading code, which is calculated beforehand, and the peak phase of the transmitting station to be demodulated is detected in consideration of the calculated auto-correlation component, thereby attaining the above object.